Filter for new files is a Feature request to ease this. Requests are backlogged. It links to some possible workarounds of scripting (example script is here). In terms of fireproof safe, consider the temperatures. Some people have off-site locations, which presumably won’t catch on fire the same time as main one.
Terminology doesn’t quite fit due to deduplication, where a new file version may share blocks with older.
If you split the backup, be sure to keep a backup job closely tied to a backup destination. Don’t mix files.
Basically, you could have an ongoing infrequent backup which every now and then gets newer changes.
Online backs up more recent changes to files, and drops old data as it ages out due to the file filtering…
Math of expansion doesn’t quite work unless it’s all new files. Existing files that expand by appends need entire file backed up. Changed file would be represented as a full set of file blocks, added a few at a time.
Also note that space reclaim from deletions is not instant. Compact has to run. It has some adjustments.
Plan is plausible, but awkward.